Wave guide modulating and switching apparatus



March 13, 1951 R, B, MUCHMORE 2,544,715

WAVE GUIDE MODULATING AND SWITCHING APPARATUS Filed Aug. 50, 1945 2 Sheets-Sheet 1 FIG-l H-q GENERATOR INVENTOR ROBERT B. MUCHMORE MZJMW Patented Mar. 13, 1951 WAVE GUIDE MODULATING AND SWITCHING APPARATUS Robert B. Muchmore, Hempstead, N. Y., assignor to The Sperry Corporation, a corporation of Delaware Application August 30, 1945, Serial No. 613,639

4 Claims.

l This invention relates generally to amplitude modulation or interruption of ultra high frequency radio waves and has reference more particularly to' electromechanical wave guide modulators or switches.

Wave guide modulators have been disclosed that are adapted to interpose a wave interceptor or reflector between juxtaposed sections of a wave guide conduit coupling a source of ultra high frequency energy to a utilization circuit for the purpose of Varying the energy now. The ow may be switched on or olf or may be modulated to any desired extent. This switching or modulation is achieved by varying the amount or position of that portion of the reector interposed between the conduit sections. This variation is usually accomplished by providing relative rotation between the conduit sections and the wave interceptor or reflector.

One of the most satisfactory wave interceptors is a simple conducting surface reflector provided with holes or windows in its surface or formed with an irregular periphery. At present the most common structure of this type employs hollow rectangular conducting wave guides whose spaced ends, forming the gap into which the recctor is interposed, are furnished with means for inhibiting radiation lost therefrom. This radiation in hibiting means may conveniently take the form of a well known choke joint or wave trap such as the one described in Practical Analysis of Ultra High Frequency, by Meagher and Markley, published by R. C. A. Service Company, Inc., 1943.

These mechanical Wave guide modulators constitute a marked advance over prior devices for the same purposein that they are mechanically rugged and not too sharply resonant and, therefore, require no critical tuning adjustments sensitive to frequency changes, but, prior to the present invention, these devices have been unduly bulky and have required a disproportionate amount of mechanical power to actuate them.

The principal object, therefore, of the present invention is to provide a simplified and improved mechanical wave guide modulator and/or switch.

A further object of the invention is to provide improved apparatus and instrumentalities embodying novel features and principles, adapted for use in realizing the above object and also adapted for use in other fields.

The invention in another of its aspects relates to novel features of the instrumentalities described hereinfor achieving the principal object of the invention and to novel principles employed in those instrumentalities, whether'or not these features and principles are used for the. said principal object or in the said field.

Another feature is the formation cfa constrice tion at the end of a wave guide associatedwitha movable wave interceptor or shutter for reducing the effective cross section of the guide and, therefore, the areaY of the interceptor or shutter necessary for substantially' blocking the guided waves. y y

A further featureis the positioning of a dia? phragm in a wave guide substantially ush with the end of the guide, the diaphragm containing a slit cut therein resonant to the frequency of the energy flowing through the guide to form a secondary source of energy having minimumdi-` mensions. Other features and advantages will become apparent as the description proceeds and after consideration of the embodiment illustrated in the drawings, wherein i Fig. 1 is an oblique View of a particular form of electromechanical wave guide modulator and switch embodying the present invention; f

Fig. 2 is a diagram illustrating the designof a wave guide diaphragm having a resonant slit formed therein; f Fig. 3 is a detailed view of a reflector disc employed in the apparatus shown in Fig. 1;

Fig. 4 is a transverse view in partial cross section of the disc and apertured diaphragm shown in Fig. 3; and

Fig. 5 is a graph illustrating the amplitude enf' velope of the waves supplied to the load by the generator shown in Fig. 1.

Similar reference numerals are employed to indicate corresponding parts in the above gures.

Generally speaking'the present invention con# templates an amplitude modulator or switch structure for ultra high frequency radio oscilla,` tions utilizing two hollow conducting wave guides having ends "disposed opposite and adjacent each other for radiating and receiving the oscillations" An abrupt constriction is provided'at the end Aof at least one of these guides for-reducing' its effective cross section, and a filter is usually assoe ciated with the guide ends for inhibiting radiae tion transversely thereof. A wave interceptor or shutter is caused to pass between these wave guide ends to modulate or block, according to the character and relative motion of the wave interceptor, the oscillations attempting to ow between the guides. The constriction may take the form of an apertured diaphragm, for exam-- ple, a metallic wall containing a rectangular slit, placed substantially flush with the wave guide end. Thisaperture or slit is designed to be reso.-

circuits nant at the operating frequency of the wave guide and may be made very narrow in comparison to the transverse dimensions of the guide.

The wave interceptor may conveniently be a reiiecting surface having a generally disc-like conguration adapted to be rotated in the gap between'ithe; guides to provide the desiredV relative motion. "The size of thisv reiiecting surface may be very substantially reduced in contrast to prior structure since, in many practical applications, the wave blocking area need' extend' only a little beyond the wave guide constriction to obtain satisfactory modulation of the .energy flow. Likewise, only slight relative transverse motion is required between the Wave' intercepting area and the wave guide constriction in order to alter radically the energy ow. This is a particularly attractive feature when square Wave switching is desired.

.Referring now to Fig. ,1, there is shown an ultrahigh frequency energy distribution system embodying the present invention that has special utility in aircraft instrument landing transmitting apparatus. In the particular structure 'chosen for illustration, wave guide switchingv and modulating-'apparatus I9 is interposed between afgenerator II and load circuits I2, I3, I4V and I5.

The apparatus I is adapted both to modulate alternately at one of two audio frequencies and distribute'the energy from generator II to loads 'connectionfwithl lwavev guides- I'I and I8, which I;

latterfarecircularly bentthrough right angles and then joined by series T junctions with wave guidesV I9, 2I-'and 22, 23, respectively.

A T jointV comprises a straight section of Wave guide from which there branches at right angles another section of waveguide. A series Tis considered that type of junction wherein'the pervpei-Tdicular*section extends from the larger dilmensionof -the guide, while a shunt T junction has the right angle section attached to the narr'ower'side ofthe guide.

Wave guide sections 2I and 22 are positioned 'side by side as are guide sections rI9 and 23. The ends of Wave guides I9 and 23 are placed opposite and adjacent but physically separated from the endsA 'o'fguides 24 and 21, respectively, the/latter being-connected to loadV circuits I2 and I4, respectively. Two air-gaps 34 and 31' are thus lformed'rof-suflicientrwidth to permit a metallic discr 28` mounted on a shaft 3! driven by a motor -321t'ob'e rotatablyrinterp'osed therein. In a similarlmanner Wave guides 2I and 22 are coupled bylair=gaps35and 36 to wave guides 25 and 26, Arespectively, feeding load circuits I5 and i3, respectively. Aidisc: 29, generally' similar to disc 281,:is also attached to the shaft 3i and is adapted to revolve between'gaps 35i.and 36.

Y-FDis'c 28,-shown in detail in'Fig. 3, has portions of its periphery ,cut avvayinorder that the gap 34i1bei alternately obstructed' and free when the motor'32is 'in'. operation. The configuration of thesdisci28, .chosen by way of example, has four teeth'33 formed' by five V-shaped notches evenly spacedy over approximately a quarter of the discs circumference. Theraction of the rotating disc 28is toip'ermit. ori impedethe-low of energy from generator I'I to wave guide 24 and thus to the load'iIZLat a frequency determined by the speed offmotorfi32 and the shape `andY placement of the teeth 33. The elise 29,` as 'shown iii'Fig.l 1, contains seven teeth formed by eight V-shaped cuts equally spaced over a quarter of the discs circumference. Disc 29 is angularly positioned on shaft 3l relative to disc 28 so that the notched portion of the former cooperates with gaps 35 and 35 only during the alternate quarter periods in vwhich fthe notched portionofl disc' 28 is clear of both gap S4 and gap 3T.

Fig. 5 illustrates the modulation envelope and transmission sequence produced by discs 28 and 29. Modulation envelopes 5I, 52, 53 and 54 correspondv to the energy applied to load circuits I2, I3, I4 and I5, respectively. Envelope 5I shows that there are ve substantially sinusoidal variationsin" the'energy to load circuit I2 during a fourth of every disc rotation and a smiliarly vmodulated portion of energy is provided to load circuit I4 during every third quarter of the cycle. Modulation envelopes 52 and 54 indicate there are eight amplitude variations in the energy transmitted' duringleachisecond and fourth quarter cycles to'loads i3 and I5, respectively. Envelopes 521 and 5t overlap slightly with adjacent quarter revolutions' oithe discs. Such an arrangement of teeth produces alternately 600 and 900 cycle modulation during alternate' quarter revolutions 'when thediscs are rotatedv at' 1800 R. P. M. It might? appear necessary to'modulate' the energy supplied to loads I3 and llseven and onehalf i 'timesiduring alternate quarter cycles, but, it

11 taina 900 cycle modulation or the modulation frequency may be appropriately altered. The instant in the modulation cycle illustrated by the position of discs 28 andA 29 in Figs. 1 and 3 is represented in Fig.v 5 bythe dashed line 50.

In order to minimize the now of energy out of the: gap -S'between vwave guide sections I9 and 24, and to permit maximum transmission between generator .H and the load circuits I2-I5, radiation in'hiI-iiting Vmeans are provided in the form' of Wave trap or choke couplings at kthe air-gaps '3d-'37. The choke coupling associatedwith 'gap' 3d, `shown in detail in Figs. 3 and 4, is preferably 'of the so-called choke-to-choke symmetrical construction comprising a pair of `annularly slotted members '38 and 39 placed on vthetends of wave guides 24 and I9, respectively. Member '33 is formed with an annular slot 4I approximately a quarter of a wavelength deep at the operating frequency of the wave guide and al quarter wavelength distant from the wave Aguide opening. Memberv 39 is similarly fashioned and cooperates with member 38, offering high impedance to energyV attempting to flow transversely of the wave guides and at the same time providing low attenuation for transmission there'along. These choke joints or wave trap couplings are well known in the art as mentioned above andV greatly enhance the efciency of the apparatus Prior to the present invention, modulating or switching discs employed with Wave guides were cumbersome being'often more than twice the diameter `of those employed here for thev same purpose. This was especially true where it was 'desired'V to modulate the energy -iiow at a relaytively high audio frequency.

Heretofore the teeth were required to be of dimensions sufiicient to overlap the guide cross section and yet the spacing between adjacent teeth was necessarily equally generous in order not to disturb the free flow oi energy. Since the required number of teeth increases with the desired modulation frequency, the speed of disc rotation remaining constant, severe windage, bulk and weight advantages were present when the desired audio frequencies were obtained.

These problems are overcome in the present apparatus by the employmentV of a transmitting diaphragm in the form of a metallic wall 42 positioned substantially ush with the end of wave guide 24. A rectangular slot 43 is symmetrically cut through the wall 42- perpendicular to the plane of polarization of the guided waves and aligned with a radius of the disc 28. The slot or window 43 may be considered as a reradiator or secondary source of energy having greatly reduced dimensions, as compared to the cross section of the wave guide. This permits a very considerable decrease in the size of the teeth necessary for substantially blocking the guided Waves.

Transmitting diaphragms with rectangular window openings are known in the art and only in combination do they form a part of t-he present invention. It has been found that such openings when suitably proportioned relative to the transverse dimensions of the guide exhibit properties of a resonant transmitting screen or source of retransmission, analogous to a parallel resonant circuit shunted across a transmission line. It has been determined empirically that the approximate dimensions for resonance with the vweight alloy with only 7 in. extreme diameters and 2 in. diameter hubs 1I. The thickness of the discs where they passed through the gaps was approximately a tenth of an inch, While the because it has -been successfully reduced to prac- TE1,0 mode of propagation are obtainable from y the relation o 2 A 2 rn/ 14%) :rr/14a) b b' construction. In the center of the cross section of the wave guide lay out a line 6| of length parallel to the longest side 62 of the guide and centered with respect to the walls of the guide. Draw branches 63 and 64 of a hyperbola passing through opposite ends of line 6l and also through corners B5. 66 and 61, E8, respectively, of the wave guide. The approximate dimensions of. a resonant rectangular opening are then any rectangle, such as opening 69, whose sides are parallel to the walls of the guide and Whose corners lie on the hyperbola. The Q of the diaphragm is found to vary inversely with the slit width.

In the embodiment of the present invention, illustrated in Figs. 1, 3 and 4, 0.04 in. was chosen as the width of the slit 43 for manufacturing convenience and to maintain the Q of the device su'iciently low so as to virtually eliminate detuning with normal variations from the nominal operating wavelength of 5.795 cm. for which the structure was designed.

The discs 28 and 29 were cast from a lighttice but there are many variations in construction which may be resorted to even when effecting the same switching and modulating cycle illustrated in Fig. 5.A For example, although the wall 42 4containing the slot 43 was placed on the output side of the gap 34, it may be placed, if desired, on the generator side. It is not, however, usual to employ diaphragms on .both sides of the gap because such an arrangement ismore subject to misadjustment.

The output wave guides 24-21 are shown in Fig. l to have a tapered section which is illustrated more clearly in Figs. 3 and 4 by reducer section i3. This reduction is only in the dimension parallel to the plane of polarization of the waves traversing the guide 24. Such reduction in wave guide dimension lowers the characteristic impedance of lthe guide without altering the phase velocity of the waves transmitted by the guide. The use of these tapered sections is optional but serves to reduce the side of the choke coupling and permits a comparable decrease in the width of the teeth 33. f

Many applications require distribution of energy between only two loads and utilize either modulated or switched energy but not both. Other uses necessitate distribution of energy between loads in pulses of the same frequency but different lengths. Such requirements are readily complied with by appropriate alteration of the conguration of the wave intercepting means, which in Fig. l comprises discs 28 and 29. A special advantage derived from the incorporation of transmitting diaphragms or resonant slits in the wave guides is the very abrupt change of energy flow between maximum and minimum with very slight movement of the wave intercepting means.

Inas'much as the present invention has very general utility in the control of ultra high fre-V quency energy and is in no way limited to specific modulation and switching sequences or rotatable disc reectors, the discussion of the same by way of illustration is not intended to restrict the breadth of the invention whose scope is set forth in the appended claims.

What is claimed is:

l. A modulator for aircraft linstrument landing systems and the like, including two pairs of output wave guides extending from respective loads to four corresponding feed points, each of said wave guides at its end adjacent the respective feed point including `a resonant iris, two branch wave guides each extending from one 0f said feed points associated with one of said pairs of output guides to the corresponding feed points 7 assooatedwi-thvthe, other pair of said output guides, `the ends ofv said branch guides being -spacedfrom thevends of said respective output guides yto form gaps, and choke coupling means at said feed points to minimize radiation from said gaps transversely thereof, series T junctions at the longitudinal centers of said branch wave guides, a further Wave guide connecting said junctions, a parallel T junction at the midpoint of said further guide, an input Wave guide `connected at one of its ends to said parallel T junction, a radioV frequency source connected to ,the other end of saidinput Wave guide, andA mov- Ysaid source to said loads.

2. The invention as set forth in claim l, Wherein said output Wave guides include tapering portions providing at said gaps a reduced dimension in theV plane of electrical polarization in the TELO mode of propagation.`

l3. The invention as set forth in claim 1, wherein said n'mvabler shutter means comprise rotatable discs provided With open sectors and extending into said gaps, and means for rotating said discs.

4. The invention as set forth in claim 3, wherein said discs are two in number, each of said discs extending into two of said gaps.

ROBERT B. MUCHMORE.

REFERENCES CITED The Vfollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,396,044 Fox Mar. 5, 1946 2,407,068 Fiske Sept. 3, 1946 2,415,242 Hershberger Feb. 4, 1947 2,423,130 Tyrrell s- June l, 1947 2,423,508 Leck July 8, 1947 2,433,368 Johnson Dec. 30, 1947 

